Acrylate pressure sensitive adhesives are well-known in the art. Ulrich (U.S. Pat. No. RE 24,906) describes alkyl acrylate copolymers, which comprise a major amount of C4 to C14 alkyl esters of acrylic acid monomers and a minor portion of a copolymerizable polar monomer such as acrylic acid. Such adhesives are widely popular due to their availability, their low cost, and their ability to provide the requisite fourfold balance of adhesion, cohesion, stretchiness, and elasticity known to be required for effective pressure sensitive adhesives.
The advantage of acrylic polymers as viscoelastic bases for pressure sensitive adhesives are well known in the art. U.S. Pat. No. RE 24,906 (Ulrich) cites many examples of these products. Initially, such compositions were made via solution polymerization. However, such methods of polymerization involved the use of large amounts of organic solvents, which may be undesirable for economic reasons.
The acrylic polymers must be in a form such that they can be coated or applied in a manner that is desirable, for example, a smooth and level coating on a film to make a tape. In this tailored structure, the polymer may lack properties for the end use such as cohesive strength, tensile strength, or modulus. To attain end use properties, cross-linking or curing is contemplated to decrease chain slippage when a stress is applied.
Current strategies to cross-link polymers in coating applications involve high energy processes such as e-beam, gamma, and ultraviolet irradiation. These processes are limited in depth of cross-linking when applied to thick layers of polymer, and use a high amount of energy.
Many of the chemical routes to cross-link polymers involve cures including polyisocyanates and polyaziridines. These cures have short pot lives, making it difficult to achieve a uniform coat before the cross-linking reaction occurs.
Recently, it has become known to form a 1,3-cyclo-addition of azides with terminal acetylene (also known as a 3+2 cycloaddition) using a copper catalyst at room temperature in what is known as a “click reaction”. Katritzky, et al., in J. Poly. Sci.: Part A, v. 46, 238-256 (2008), describe the preparation and characterization of end-capped azides and alkynes, wherein the azides were combined in 1,3-dipolar cycloaddition reactions to form triazole linked polymers.
However, U.S. Pat. No. 5,681,904 (Manzara), has taught away from cross-linking azides with activated acetylene (i.e., acetylene linked directly to a carbonyl) because such reactions are relatively fast and would lead to a short pot life.